By virtue of excellent magnetic properties, Nd—Fe—B permanent magnets find an ever increasing range of application. The recent challenge to the environmental problem has expanded the application range of these magnets from household electric appliances to industrial equipment, electric automobiles and wind power generators. It is required to further improve the performance of Nd—Fe—B magnets.
Indexes for the performance of magnets include remanence (or residual magnetic flux density) and coercive force. An increase in the remanence of Nd—Fe—B sintered magnets can be achieved by increasing the volume factor of Nd2Fe14B compound and improving the crystal orientation. To this end, a number of modifications have been made on the process. For increasing coercive force, there are known different approaches including grain refinement, the use of alloy compositions with greater Nd contents, and the addition of effective elements. The currently most common approach is to use alloy compositions having Dy or Tb substituted for part of Nd. Substituting these elements for Nd in the Nd2Fe14B compound increases both the anisotropic magnetic field and the coercive force of the compound. The substitution with Dy or Tb, on the other hand, reduces the saturation magnetic polarization of the compound. Therefore, as long as the above approach is taken to increase coercive force, a loss of remanence is unavoidable. Since Tb and Dy are expensive metals, it is desired to minimize their addition amount.
In Nd—Fe—B magnets, the coercive force is given by the magnitude of an external magnetic field created by nuclei of reverse magnetic domains at grain boundaries. Formation of nuclei of reverse magnetic domains is largely dictated by the structure of the grain boundary in such a manner that any disorder of grain structure in proximity to the boundary invites a disturbance of magnetic structure, helping formation of reverse magnetic domains. It is generally believed that a magnetic structure extending from the grain boundary to a depth of about 5 nm contributes to an increase of coercive force. It is difficult to acquire a morphology effective for increasing coercive force.
The documents pertinent to the present invention are listed below.
Patent Document 1:JP-B 5-31807Patent Document 2:JP-A 5-21218Non-Patent Document 1:K. D. Durst and H. Kronmuller,“THE COERCIVE FIELD OF SINTEREDAND MELT-SPUN NdFeB MAGNETS,”Journal of Magnetism and MagneticMaterials, 68 (1987), 63-75Non-Patent Document 2:K. T. Park, K. Hiraga and M.Sagawa, “Effect of Metal-Coatingand Consecutive Heat Treatmenton Coercivity of Thin Nd—Fe—BSintered Magnets,” Proceedingsof the Sixteen InternationalWorkshop on Rare-Earth Magnetsand Their Applications, Sendai,p. 257 (2000)Non-Patent Document 3:K. Machida, H. Kawasaki, S.Suzuki, M. Ito and T. Horikawa,“Grain Boundary Tailoring ofNd—Fe—B Sintered Magnets andTheir Magnetic Properties,”Proceedings of the 2004 SpringMeeting of the Powder & PowderMetallurgy Society, p. 202